Multicore

[arcz]

To test, add --workers N to echidna command or workers: N to config.
This PR also adds --timeout <seconds> to CLI which was available only from the yaml config.

Notable code changes:

• Campaign type is now WorkerState with less data. Tests, coverage and corpus are now IORefs that live in Env and the data is shared between all workers. The contention on this data is minimal as modifications rarely happen besides the very beginning when coverage grows rapidly.
• There is a shared event queue that uses Chan. Workers push events when something interesting happens. Those events drive the UIs.
• UI module has been reworked to work with multiple workers that can stop at different times.
• Open tests don't count calls, it was the same for all open tests and was moved to WorkerState. The call number is now displayed at the top of TUI.
• The number of functions that use IO has grown because it's easier to grab data straight from Env. This is something that can be improved in the future.

Things to think about:

• At the moment, all workers replay the corpus but this is redundant work. Either split corpus across workers to replay or replay it first and then start workers. Consider calls to external contracts that might not be yet deployed. Relevant Show a proper error message when an invalid corpus is used #1008 Corpus is now chunked and distributed across workers to replay.
• There is a lot of contention while shrinking because all workers try to shrink. This can be improved by marking Large test with a workerId that falsified the test and only this worker shrinks the test.

Closes #476